GROUND-ZERO

ACL2 concludes its initialization (boot-strapping) procedure by
defining the theory ground-zero; see theories. In fact, this
theory is just the theory defined by (current-theory :here) at the
conclusion of initialization; see current-theory.

Note that by executing the event

(in-theory (current-theory 'ground-zero))

you can restore the current theory to its value at the time you started up
ACL2.